Compression Testing - Page 2 - PeachParts Mercedes-Benz Forum

t walgamuth · #1 02-05-2009, 05:41 PM

My book says that you need 220psi for reliable starting measured cold.

I am not sure why such a big deal is made of testing them hot. When you really need to know how they are is when you want to start them from cold.

jt20 · #2 02-06-2009, 12:55 PM

Quote:

Low compression on a cold engine is a major cause for hard starts or no starting. Once the engines warm it always starts easier. So wouldn't a cold compression check give you a better idea of the state of the engine.

no.

you want to measure the running performance of the engine and it's efficiency capabilities. ie- warm.

Biodiesel300TD · #3 02-06-2009, 02:37 PM

Quote:

Originally Posted by jt20

no.

you want to measure the running performance of the engine and it's efficiency capabilities. ie- warm.

But warm compression doesn't help on a cold start. Cold compression is what counts on a cold start. And I would argue, it is a better indicator of good compression. Good cold compression will translate into great warm compression. When one of us goes to look at a car we might buy, we usually want to start the car cold, and are disappointed when the engine is warm. Just about any warm diesel will start. But a cold diesel is much harder to start. I've experienced it myself. Having a hard time getting the old girl started on those cold mornings, but once she's been run she'll fire up quite easily.

jt20 · #4 02-06-2009, 02:44 PM

one should seriously consider rebuilding the head if that is the case.

If all one is concerned about is whether an engine will start after a rebuild.. then by all means, do a cold test.

You never now what might happen to an old engine once it is warmed up.

Oracle12345 · #5 02-06-2009, 10:31 PM

as for the compression test when the engine is cold argument its simply this: If you have trouble starting your car when its cold or it doesnt start at a certain temperature then you can be sure that the engine compression is a factor to consider. Other factors to cause hard starting is problems with the fuel system.

Why waste your time on a cold engine compression test when the car is telling you something?

as for the orginal poster if the car started fine when it was cold out then i almost wouldnt bother with the test. if you had head the shaved then I would bother.